1. Field of the Invention
This invention relates to fuel ignition systems of the direct ignition type, and more particularly, to a control circuit for use in such systems for providing an interlock which prevents start-up under certain failure conditions.
2. Description of the Prior Art
In known fuel ignition systems of the direct ignition type for use in heating systems, a fuel valve is tentatively operated during a trial for ignition interval in response to the closing of thermostatically controlled contacts, permitting the valve to supply fuel to a burner for ignition to establish a flame. A flame sensing circuit detects the flame and maintains the valve in fuel supplying condition, the valve being deenergized in the event the fuel fails to be ignited within the trial for ignition interval.
Typically, the operation of the fuel valve is controlled by a relay of the flame sensing circuit which has normally open contacts connected in the energizing path for the valve. When energized, the relay closes its contacts to connect the valve to an energizing circuit to permit the valve to operate. When the thermostatically controlled contacts open, the fuel valve is deenergized to interrupt the supply of fuel to the burner whereby the flame is extinguished. The flame sensing circuit responds to the loss of flame to deenergize the relay which opens its contacts to disconnect the valve from the energizing circuit in preparation of the next ignition cycle.
However, should the relay contacts which control the energization of the valve become welded together, following a successful ignition cycle, or for a circuit failure which permits the relay of the flame sensing circuit to remain energized in the absence of a flame, the valve remains connected to the energizing circuit and will be energized the next time the thermostatically controlled contacts close, and will remain energized even though the fuel fails to be ignited, permitting fuel to emanate from the burner, unlit, an undesirable condition.
Also, many systems employ an RC timing circuit to define the trial for ignition interval. In such timing circuits, the timing is changed, that is increased, if the capacitor becomes leaky or if the resistance increases as by bad solder joints. An increase in the trial for ignition period resulting from a change in the timing circuit permits unburned fuel to emanate from the fuel outlet for a longer time, a potentially hazardous condition.